1. Field of the Invention
The present invention relates to a thermal transfer sheet and, particularly, to a thermal transfer sheet which has high sensitivity, is adaptable to high-speed printing and has high abrasive resistance and printing quality.
2. Description of the Related Art
A melting type thermal transfer system has been known, in which an applicable thermal transfer sheet composed of a substrate sheet and a coloring layer which is obtained by dispersing a coloring agent such as a pigment or a dye in a heat-meltable binder such as wax or a resin and supported by the substrate sheet such as a plastic film, and energy corresponding to image information is applied to the thermal transfer sheet by means of a heating device such as a thermal head, and thus the coloring agent is transferred from the thermal transfer sheet to an image-receiving sheet such as paper or a plastic sheet together with the binder. A print image formed by this melting type thermal transfer system has high density and superior sharpness and is therefore suitable for the recording of binary images such as characters and line drawings. In addition, when plural coloring layers such as yellow, magenta, cyan and black are printed and recorded on the image-receiving sheet in a superposed manner with the use of one or more thermal transfer sheets, a multicolor or full color image can be formed by subtractive color mixing.
When images such as a bar code for which strict standards as to line width, intervals between lines and length are established are printed by melting transfer system, it is required for a thermal transfer sheet to be provided with a coloring layer having good film-cuttability and to have sharp printing ability. Also, it is required for a print product to have abrasive resistance in order to enable exact reading by a bar code reader and prevent the print portion from being soiled by rubbing in the handling of the print product. For this, many types of thermal transfer sheet provided with a coloring layer primarily comprising a resin such as an acrylic resin or vinyl acetate type resin have been proposed.
When information such as a bar code is recorded by thermal transfer in a transfer-receiving material by using a thermal transfer sheet of the melting type transfer system, high-speed processing (high-speed printing) is required in many applications. In this case, particularly a thermal transfer printer mounted with a so-called end face type thermal head, specifically, provided with a heat generating resistance part at an end surface of a substrate perpendicular to the plane on which a driver IC for driving is disposed is frequently used.
However, as the thermal transfer sheet used to attain high-speed processing (high-speed printing) in the above thermal transfer recording, one utilizing a material having high sensitivity is used and a thermal transfer print product prepared using such a thermal transfer sheet provided with a coloring layer made of a highly sensitive resin poses the problem that it is difficult to provide functions as the print product, namely, durability such as wear resistance and heat resistance because the coloring layer such as those which are easily melted and softened at a heating temperature during printing is transferred.
On the other hand, attempts have been made to dispose a peelable layer and the like in combination with the coloring layer on a substrate of a thermal transfer sheet, allowing a print portion to have a multilayer structure to improve the durability, such as wear resistance and heat resistance of the print product. For example, in these attempts, a coloring layer primarily comprising a thermoplastic resin for the purpose of providing the print product with durability such as wear resistance and heat resistance after printing and an adhesive layer using a material having low softening point to provide adaptability to high-speed transfer are disposed on a substrate.
However, in actual, the coloring layer primarily comprising a heat-resistant resin having high softening point has difficulties in conducting thermal energy to the adhesive layer when high-speed printing is carried out by heating from the backface of the thermal transfer sheet. A material having a low softening point and high sensitivity is consequently used for formation of the adhesive layer, which easily causes the so-called blocking phenomenon and background soiling. In the blocking phenomenon, the adhesive layer side of the thermal transfer sheet adheres to the backface side of the substrate of the thermal transfer sheet in the rolled condition during storage. The background soiling is caused by rubbing of the thermal transfer sheet against image-receiving paper during printing.
Accordingly, it is an object of the present invention to solve the aforementioned problems by providing a thermal transfer sheet which is superior in storage stability in a rolled condition and in printing quality, and also capable of forming a print products having excellent functions as to durability such as wear resistance and heat resistance in high-speed printing.
In order to attain the above object, a thermal transfer sheet according to the present invention comprises a substrate, a coloring layer and an adhesive layer, the coloring layer and the adhesive layer being disposed on one side of the substrate in this order from a side near the substrate, wherein the adhesive layer is formed of a mixture comprising a polymerization product (A) obtainable by polymerizing an xcex1-olefin/a maleic acid anhydride copolymer with a maleic acid anhydride monoester and an ethylene/vinyl acetate copolymer (B).
Since the above polymerization product A is contained in the adhesive layer, particularly blocking during storage in a rolled condition can be prevented. Also, since the above copolymer B is contained in the adhesive layer, the thermal transfer sheet has high printing quality in, particularly, high-speed printing.
Further, since the copolymerization product A and the copolymer B are blended to form the adhesive layer, a thermal transfer sheet which is superior in storage stability in a rolled condition and in printing quality, and also capable of forming a print products having excellent functions as to durability such as wear resistance and heat resistance in high-speed printing and is well-balanced between various qualities can be provided.
Preferably said adhesive layer further comprises a polyethylene resin filler (C). Blending of the polyethylene resin filler (C) further improves transfer characteristics in high-speed printing.
A peelable layer comprising wax as its major component is preferably disposed between said coloring layer and said substrate. The wax melts during thermal transfer and improves the peelability of the coloring layer from the substrate and after transferred, at least a part thereof is transferred together with the coloring layer to cover the transferred image, and it imparts good lubricity to a protective layer of the coloring layer or particularly the transferred image, whereby the abrasive resistance of the transferred image can be more improved.
The thermal transfer sheet of the present invention is preferably used in a printing system using a thermal transfer printer provided with an end face type thermal head. This makes it possible to carry out thermal transfer recording in high-speed printing.